The present invention relates to an apparatus for printing information on a label or other various indicator materials, and more particularly to a label printing apparatus for printing information such as a production date of a content in a container (e.g., bottle) and a lot number thereof on a label to be attached to the container.
A conventional labeling machine used for attaching a label onto a container such as a bottle is provided with, for example, a cylinder having on its peripheral surface a plurality of vacuum holders to transfer labels by means of vacuum suction. The peripheral surface of the cylinder is provided with a label magazine for feeding labels in a given direction, for example, in the right rotation direction or the left rotation direction, a laser printing device for printing information such as a lot number and a production date on the label, and a paste application device for applying paste to the label held by the vacuum holder by means of a rotating roller.
The vacuum holder provided on the peripheral surface of the cylinder rotates in accordance with the rotation of the cylinder and receives the label from the label magazine by means of vacuum suction. Then, the back surface of the label is applied with paste by the paste application device while the label is held by the vacuum holder. The vacuum holder transfers the label to a bottle which is conveyed by a belt conveyer, and attaches the label onto the side surface of the bottle.
Onto a driving means for rotating the cylinder, an absolute encoder that outputs data of the rotation absolute position of the cylinder is fitted. Based on the data of the rotation absolute position output from the absolute encoder, the laser printing device prints various information such as a production date, a lot number and a factory number on the label transferred by the vacuum holder.
However, the absolute encoder for detecting the absolute position, which is used for the above-described conventional labeling machine, has a scatter of about 0.5 msec in the response time. Supposing labels each having a size of about 5 cm.times.6 cm are attached at a rate of 1,000 sheets/min, the relative moving rate of the label to the printing position of the laser printing device would be about 2 mm/msec. This means that a deviation of about 2 mm arises in the printing position because the printing starting time is delayed 1 msec.
Therefore, the printing position deviates by about 1 to 2 mm due to the scatter in the response time of the absolute encoder.
In the above-mentioned labeling machine, detected is not a real position of the label but a position of the cylinder for holding and transferring the label. For this reason, if a deviation is present between the detected position of the cylinder and the real position at which the label is held, printing in the correct position of the label is difficult. Moreover, even when the vacuum holder does not hold a label, the printing operation is carried out by the laser, thereby to cause waste of electric power and damage of the vacuum holder. For avoiding such disadvantage, it is necessary to conduct a complicated control, resulting in a new problem.
U.S. Pat. No. 4,844,769 discloses a labeling machine provided with a printing monitor. This labeling machine includes plural pallets rotating along a predetermined track on a pallet rotor, a paste application device arranged in the direction of the pallet's rotation, a label magazine, a printing device, and a print-reading detection head. The print-reading detection head (for example, constructed by a camera) discriminates whether printing on the label has been done or not by the printing device.
In the conventional labeling machine described above, however, the real position of the label is not detected, and therefore there resides a problem of difficult printing in the correct position on the label. Moreover, there is other problem that printing is made even when the pallet does not hold the label.
By the way, the above-mentioned laser printing is mainly carried out by irradiating only the necessary portion of a substrate surface with a laser beam to heat that portion of the substrate so as to modify or remove the portion, or by irradiating a film coated on a substrate surface with a laser beam to remove only the film so as to form a contrast between the laser-irradiated portion (printed portion) and the unirradiated portion (ground portion).
However, in the method of removing the printing ink for example, a high contrast between the substrate paper and the printing ink is necessary, and hence sharp printing on a pale white label is impossible. Further, the removal of the printing ink (material destruction) causes roughening of a border between the irradiated portion and the unirradiated portion, whereby accurate printing cannot be obtained. On that account, use of heat-sensitive color development by means of irradiation of a label having a paper substrate with a laser beam, that is non-destructive type color development, has been proposed, and a variety of applied examples are known in literatures (e.g., Japanese Patent Laid-Open Publications No. 52442/1978, No. 11857/1980 and No. 148695/1984), but none of them have been practically used yet.